1. Technical Field
The invention relates to fabrication of devices built to submicron design rules. Plasma-derived x-ray radiation serves for pattern delineation for small features considered unattainable by use of longer wavelength electromagnetic radiation. The plasma source is matched to a projection camera operating in a ringfield scanning mode. Very Large Scale Integration ("VLSI") is a prime objective.
2. Description of the Prior Art and of Co-pending U.S. patent application Ser. No. 08/059,924 filed May 10, 1993.
State-of-the art VLSI is a 16 megabit chip with circuitry built to design rules of 0.5 .mu.m. Effort directed to further miniaturization takes the initial form of more fully utilizing resolution capability of presently-used ultraviolet ("UV") delineating radiation. "Deep" UV (.lambda.=0.3 .mu.m-0.1 .mu.m), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.25 .mu.m or slightly smaller.
At still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. An extensive effort depends on electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
Two x-ray radiation sources are under consideration. The first, the electron storage ring synchrotron, has been used for many years and is at advanced stage of development. Electrons, accelerated to relativistic velocity, in following their magnetic-field-constrained orbit, emit x-ray radiation. Radiation, in the wavelength range of consequence for lithography, is reliably produced. The synchrotron produces precisely defined radiation to meet the demands of extremely sophisticated experimentation but is a large, very costly piece of apparatus.
Plasma x-ray sources are less costly. These depend upon a high power, pulsed laser--e.g. an yttrium aluminum garnet (YAG) laser, or an excimer laser, delivering 500-1,000 watts of power to a 50 .mu.m-250 .mu.m spot, thereby heating a source material to e.g. 250,000.degree. C., to emit x-ray radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant).
A variety of x-ray patterning approaches are under study. Probably the most developed is proximity printing. In this approach 1:1 imaging is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer. This spacing lessens likelihood of mask damage, but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in between the mask and the wafer necessitates a high level of parallelicity in the incident radiation. X-ray radiation of wavelength .lambda..ltoreq.16 .ANG. is required for 0.25 .mu.m patterning to limit scattering at feature edges.
Use has been made of the synchrotron in proximity printing. Relatively small power resulting from the 10 mrad-20 mrad arc of collection, together with the high-aspect ratio of the emission fan, has led to use of a scanning high-aspect ratio illumination field (rather than to full-field imaging).
Projection lithography has natural advantages over proximity printing. Camera optics in between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of "soft x-ray" in the .lambda.=100 .ANG.-200 .ANG. wavelength range increases the permitted angle of incidence for glancing-angle optics. The resulting system is known as soft x-ray projection lithography (SXPL).
A favored form of SXPL is ringfield scanning. The long narrow illumination field is arc-shaped rather than straight, with the arc being a segment of the circular ring with its center of revolution at the optic axis of the camera. Use of such an arcuate field avoids radially-dependent image aberrations in the image. Use of object:image reduction of e.g. 5:1 results in significant cost reduction of the, now, enlarged-feature mask.
Co-pending U.S. patent application, Ser. No. 08/059,924 filed May 10, 1993 describes and claims device fabrication using synchrotron derived x-ray radiation. SXPL is one form of lithography described and claimed.
It is expected that effort toward adaptation of plasma x-ray sources for SXPL will continue. Design of collection and processing optics--design of the condenser-is complicated by the severe mismatch between the plasma emission pattern and that of the ringfield scan line. A typical plasma x-ray source has a 1:1 aspect ratio emission pattern. The required scan line is likely greater than 10:1.